The present invention generally relates to a fluid rotating apparatus, and more particularly to a fluid rotating apparatus, such as a positive displacement vacuum pump used in the semiconductor manufacturing facilities to vacuumize a chamber, in the structure to synchronously rotate a plurality of rotary shafts rotating at high speeds.
A CVD device, a dry etching device, a sputtering device, a vapor deposition device, etc. used in the manufacturing process of semiconductors requires a vacuum pump to produce a vacuum environment. A vacuum pump is also used when magnetic discs, liquid crystals, and the like are manufactured.
In a positive displacement vacuum pump, two rotors are synchronously rotated while the screws provided in the outer peripheries of the rotors are meshed with each other. At this time, as the volume of a space formed between the two screws is changed, this change of the volume is utilized to suck and compress gas thereby to vacuumize the chamber.
An example of the aforementioned screw type vacuum pump is illustrated in FIG. 12. There are provided two rotors in a housing 311, having the respective rotary center shafts set parallel to each other. Screws are formed in the outer peripheries of the rotors 312a, 312b. As recessed parts (grooves) 313a of the screw of one rotor are meshed with projecting parts 313b of the other rotor, a space is defined between the two rotors. Therefore, when the rotors 312a, 312b rotate, the volume of the space is changed, thereby achieving suction/discharge of the air.
The drawbacks of the above screw type vacuum pump will be discussed now.
In the positive displacement vacuum pump shown in FIG. 12, the rotors 312a, 312b are synchronously rotated by the action of timing gears. More specifically, the rotation of a motor 315 is transmitted from a driving gear 316a to an intermediate gear 316b and further to one of timing gears 316c, 316d which are provided in the shafts of the rotors 312a, 312b and meshed with each other. The phase of the rotating angle of each rotor 312a, 312b is adjusted through the engagement of the timing gears 316c, 316d. That is, both the transmission of power from the motor and the synchronous rotation of the rotors are realized by means of gears in the vacuum pump of this screw type, and therefore a lubricating oil filled in a mechanical operating chamber 317 housing the gears should be supplied to the gears. Moreover, a mechanical seal 319 is provided between the mechanical operating chamber 317 and a fluid operating chamber 318 where the rotors are accommodated so as to prevent invasion of the lubricating oil.
As depicted hereinabove, the screw type vacuum pump with two rotors are inconvenient in the following points:
(1) Many gears are required for the transmission of power and the synchronous rotation of rotors, thus increasing the number of components and making the apparatus complicate in structure;
(2) The contact-type synchronous rotation of rotors using gears is unable to operate at high speeds and is bulky;
(3) A mechanical seal should be regularly exchanged with a fresh one subsequent abrasion, and is not completely maintenance-free;
(4) The large sliding torque due to the mechanical seal increases the mechanical loss.